The present invention relates in general to surgical instruments and surgical techniques. More particularly, the present invention is directed to a surgical tool for transplanting sheets of retinal cells, epithelial tissue and/or choroidal tissue in a volute configuration through a standard-sized incision in the eye, a graft for transplantation to the subretinal region of the eye, a method for preparing such grafts for transplantation and a method for reconstructing dystrophic retinas, retinal pigment epithelial layers and choroids.
The retina is the sensory epithelial surface that lines the posterior aspect of the eye, receives the image formed by the lens, transduces this image into neural impulses and conveys this information to the brain by the optic nerve. The retina comprises a number of layers, namely, the ganglion cell layer, inner plexiform layer, inner nuclear layer, outer plexiform layer, outer nuclear layer, photoreceptor inner segments and outer segments. The outer nuclear layer comprises the cell bodies of the photoreceptor cells with the inner and outer segments being extensions of the cell bodies.
The choroid is a vascular membrane containing large branched pigment cells that lies between the retina and the sclerotic coat of the vertebrate eye. Immediately between the choroid and the retina is the retinal pigment epithelium which forms an intimate structural and functional relationship with the photoreceptor cells.
Several forms of blindness are primarily related to the loss of photoreceptor cells caused by defects in the retina, retinal pigment epithelium, choroid or possibly other factors (e.g. intense light, retinal detachment, intraocular bleeding). In several retinal degenerative diseases select populations of cells are lost. Specifically, in macular degeneration and retinitis pigmentosa, the retinal photoreceptors degenerate while other cells in the retina as well as the retina""s central connections are maintained. In an effort to recover what was previously thought to be an irreparably injured retina, researchers have suggested various forms of grafts and transplantation techniques, none of which constitute an effective manner for reconstructing a dystrophic retina.
The transplantation of retinal cells to the eye can be traced to a report by Royo et al., Growth 23: 313-336 (1959) in which embryonic retina was transplanted to the anterior chamber of the maternal eye. A variety of cells were reported to survive, including photoreceptors. Subsequently del Cerro was able to repeat and extend these experiments (del Cerro et al., Invest. Ophthalmol. Vis. Sci. 26: 1182-1185, 1985). Soon afterward Turner, et al. Dev. Brain Res. 26:91-104 (1986) showed that neonatal retinal tissue could be transplanted into retinal wounds.
In related studies, Simmons et al., Soc. Neurosci. Abstr. 10: 668 (1984) demonstrated that embryonic retina could be transplanted intracranially, survive, show considerable normal development, be able to innervate central structures, and activate these structures in a light-dependent fashion. Furthermore, these intracranial transplants could elicit light-dependent behavioral responses (pupillary reflex) that were mediated through the host""s nervous system. Klassen et al., Exp. Neurol. 102: 102-108 (1988) and Klassen et al. Proc. Natl. Acad., Sci. USA 84:6958-6960 (1987).
Li and Turner, Exp. Eye Res. 47:911 (1988) have proposed the transplantation of retinal pigment epithelium (RPE) into the subretinal space as a therapeutic approach in the RCS dystrophic rat to replace defective mutant RPE cells with their healthy wild-type counterparts. According to their approach, RPE was isolated from six- to eight-day old black eyed rats and grafted into the subretinal space by using a lesion paradigm which penetrates through the sclera and choroid. A 1 ml injection of RPE (40,000-60,000 cells) was made at the incision site into the subretinal space by means of a 10 ml syringe to which was attached a 30 gauge needle. However, this method destroys the cellular polarity and native organization of the donor retinal pigment epithelium which is desirable for transplants.
del Cerro, (del Cerro et al., Invest. Ophthalmol. Vis. Sci. 26: 1182-1185, 1985) reported a method for the transplantation of tissue strips into the anterior chamber or into the host retina. The strips were prepared by excising the neural retina from the donor eye. The retina was then cut into suitable tissue strips which were then injected into the appropriate location by means of a 30 gauge needle or micropipette with the width of the strip limited to the inner diameter of the needle (250 micrometers) and the length of the strip being less than 1 millimeter. While del Cerro reports that the intraocular transplantation of retinal strips can survive, he notes that the procedure has some definite limitations. For instance, his techniques do not allow for the replacement of just the missing cells (e.g. photoreceptors) but always include a mixture of retinal cells. Thus, with such a transplant appropriate reconstruction of the dystrophic retina that lacks a specific population of cells (e.g., photoreceptors) is not possible.
del Cerro et al., Neurosci. Lett. 92: 21-26, 1988, also reported a procedure for the transplantation of dissociated neuroretinal cells. In this procedure, the donor retina is cut into small pieces, incubated in trypsin for 15 minutes, and triturated into a single cell suspension by aspirating it through a fine pulled pipette. Comparable to the Li and Turner approach discussed above, this procedure destroys the organized native structure of the transplant, including the donor outer nuclear layer; the strict organization of the photoreceptors with the outer segments directed toward the pigment epithelium and the synaptic terminals facing the outer plexiform layer are lost. Furthermore, no means of isolating and purifying any given population of retinal cells (e.g. photoreceptors) from other retinal cells was demonstrated.
It is believed by the present inventor that it is necessary to maintain the photoreceptors in an organized outer nuclear layer structure in order to restore a reasonable degree of vision. This conclusion is based on the well known optical characteristics of photoreceptors (outer segments act as light guides) and clinical evidence showing that folds or similar, even minor disruptions in the retinal geometry can severely degrade visual acuity.
Among the objects of the present invention, therefore, may be noted the provision of a method which conserves relatively large expanses of tissue harvested from a donor eye; the provision of such a method in which a relatively large expanse of harvested tissue is so formed as to enable the harvested tissue to be inserted into a standard-sized incision in the eye; the provision of such a method in which the polarity and organization of the cells at the time of harvest are maintained in the graft; and the provision of a method for implantation of grafts to the subretinal area of an eye.
Further among the several objects and features of the present invention may be noted the provision of a graft for use in the reconstruction of a dystrophic retina or rescue of endogenous photoreceptor cells of an individual afflicted with an inherited or acquired retinal disease which causes a progressive loss of rods and subsequent eventual cone dystrophy, dysfunction and/or loss; the provision of such a graft which facilitates regrowth of photoreceptor axons by maintaining the polar organization of the photoreceptor and the close proximity of their postsynaptic targets with the adjacent outer plexiform layer upon transplantation.
Further among the several objects and features of the present invention may be noted the provision of a surgical tool for use in the implantation method which forms the graft for insertion into a standard-sized incision; and the provision of a surgical tool for use in the transplantation method which allows appropriate retinotopic positioning and which protects photoreceptors, retinal pigment epithelial tissue, choroidal tissue and/or Bruch""s membrane from damage prior to and as the surgical device is positioned in the eye.
Generally, the implantation method comprises coiling an implantable material which is of sheet-like form to form a volute. The convolutions of the volute are free of one another for subsequent uncoiling of the implantable material substantially to its original sheet-like form. An incision is made in the host eye for the insertion of the volute to a position between the retina and the underlying tissue of the host eye. The incision is smaller than the incision that would be required for insertion of the implantable material in its uncoiled sheet-like form. The volute is inserted one end first into the host eye through the incision to a position between the retina and the underlying tissue. The volute uncoils after its insertion to lie in sheet-like form between the retina and the underlying tissue of the host eye and the incision is closed.
Generally, the graft for implantation comprises a layer of a non-toxic flexible composition which substantially dissolves at body temperature and a material to be implanted coiled to form a volute. The volute is insertable one end first through the incision dimensioned in accordance with the cross-sectional area of the volute to a position for implantation, and then uncoiled to lie in sheet-like form at the site of implantation.
Generally, the implement for the formation of a volute comprises a tubular body open at one end and having a funnel. A carrier enters one end first in the tubular body at the open end thereof and is fed along the body into and through the funnel. The engagement of the carrier as it is fed through the funnel with an interior surface of the funnel causes the carrier to coil into the volute. The volute exits from a small end of the funnel.
Other objects and features of the invention will be in part apparent and in part pointed out hereinafter.